1. Field of the Invention
The invention relates to a line-narrowed laser, and particularly to a tunable excimer or molecular fluorine laser having a thermally and mechanically stabilized grating.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. The ArF and KrF lasers have a broad characteristic bandwidth of 300 to 400 pm or more (FWHM). Vacuum UV (VUV) uses the F2-laser which characteristically emits two or three closely spaced lines around 157 nm.
It is important for their respective applications to the field of sub-quarter micron silicon processing that each of the above laser systems become capable of emitting a narrow spectral band of known bandwidth and around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm (for ArF and KrF lasers) for use with all-reflective optical imaging systems, and for catadioptric imaging systems to less than 0.6 pm, and preferably less than 0.5 pm, are being continuously improved upon.
For the application of excimer lasers as light sources for steppers and/or scanners for photographic microlithography, it is desired to have laser emission within a range that is far smaller than the natural linewidth, e.g., around 300 to 400 pm for ArF and KrF lasers. The extent of the desired line narrowing depends on the imaging optics of the stepper/scanner devices. The desired bandwidth for catoptric systems is less than around 50 pm, and for catadioptric or dioptric optics it is less than around 0.6 pm. Currently, used systems for the KrF laser emitting around 248 nm have a bandwidth around 0.6 pm. To improve the resolution of the projection optics, a narrower laser bandwidth is desired for excimer laser systems of high reliability and very small bandwidth of 0.4–0.5 pm or less.
A line-narrowed excimer or molecular fluorine laser used for microlithography provides an output beam with specified narrow spectral linewidth. It is desired that parameters of this output beam such as wavelength, linewidth, and energy and energy dose stabilty be reliable and consistent. Narrowing of the linewidth is generally achieved through the use of a linewidth narrowing and/or wavelength selection and wavelength tuning module (hereinafter “line-narrowing module”) most commonly including prisms, diffraction gratings and, in some cases, optical etalons.
U.S. patent application Ser. No. 09/317,527, which is assigned to the same assignee as the present application and is hereby incorporated by reference, describes the use of a pressure-tuned etalon (see also U.S. Pat. Nos. 5,901,163 and 4,977,563, also hereby incorporated by reference). The etalon is enclosed within a housing and an inert gas is filled particularly between the plates forming the etalon gap. The interferometric properties of the etalon are controlled by adjusting the pressure of the gas, and thus the index of refraction of the gas in the gap.
Line-narrowing modules typically function to disperse incoming light angularly such that light rays of a beam with different wavelengths are reflected at different angles. Only those rays fitting into a certain “acceptance” angle of the resonator undergo further amplification, and eventually contribute to the output of the laser system.
For broadband excimer lasers such as the ArF and KrF lasers mentioned above, the central wavelengths of line-narrowed output beams may be tuned within their respective characteristic spectra. Tuning is typically achieved by rotating the grating or highly reflective (HR) mirror associated with the line-narrowing module. The grating is, however, a fairly bulky optical component resulting in difficulties for precision tuning.
Excimer and molecular fluorine lasers particularly manufactured for photolithography applications are being developed to emit pulsed radiation at higher repetition rates such as 1–2 kHz and above. At these higher repetition rates, improvements are sought for reducing thermal stresses on the resonator optics.